The energy that is associated with the disordered, random movement of the molecules. Internal energy is referred to the unseen microscopic energy at the atomic and the molecular level. Internal energy is commonly denoted by U.
Internal Energy and WaterWhen a glass or a beaker full of water at room temperature is kept on a table, grossly at the macroscopic level, there is no energy either kinetic or potential, but at the microscopic level, a large number of molecules are seen moving at a rapid speed. This microscopic kinetic energy is determined to be a part of internal energy. When the glass of water is placed on an elevated surface, the gravitational potential energy between the glass and the surrounding is elevated.
Since the internal energy is dependent on the potential energy that is generated as a result of the interactions between the surroundings and the system, there is no change in the internal energy of water.Let us try to compare the similarities and differences between 1g of water and 1g of copper at 0oC.Though the kinetic energy is the same for both, the potential energy differs, resulting in a change in the internal energy between the two. Since the change in the kinetic energy is proportional to the change in the temperature, at constant temperature, the kinetic energy does not change but the total energy required at the potential energy portion is much higher resulting in a higher specific heat and hence the internal energy differs between the two since internal change is a sum of the kinetic energy and the potential energy.
For example, take two containers with water that are identical enough and are maintained at the same temperature. Increase the temperature by one degree in one of the flasks by keeping it on a flame and stir vigorously the second container such that the same temperature change occurs. One container of water contains more heat and one container has more work. By the measurement of the difference in the temperature, the internal energy of the water within the container can be measured.Conclusion for Internal Energy of WaterTo conclude, internal energy, which is the total sum of the kinetic and the potential energies, remains unchanged when the water is in stable condition with no motion even with increased gravitational potential energy.